Nb3Sn is a brittle compound that must be formed in situ at its final size through a heat treatment during the course of which the tin reacts with the niobium. Two processes are currently used to manufacture Nb3Sn, The most popular process to form the conductor is the bronze process in which filaments of Nb are drawn down in a matrix of CuSn, bronze. During the following heat treatment, the tin diffuses through the bronze to produce Nb3Sn while reacting with niobium. The second process is the Internal Tin process in which the tin is incorporated in elemental form, often with additives, and then heat-treated to diffuse the tin and react the Nb.
Nb3Sn superconductor wire is fabricated from a large number of metallic filaments. Nb3Sn is an intermetallic compound having a well-defined stoichiometry, typically obtained by treatment by high heat for an extended period of time. These compounds are important in industry because of their superior high field properties. The most important criterion in determining the usefulness and quality of the Nb3Sn superconductor is Jc, which depends on the conductor composition. The critical current densities in multifilament Nb3Sn superconductors are increased as the ratio of Nb3Sn to matrix increases in the superconductor. Filament uniformity and the amount of superconductor in the wire are the most important parameters. The inherent quality of the Nb3Sn which is effected by specific dopants to enhance the grain size of the material also has an effect on current density values.
The preferred technique used to fabricate high current density Nb3Sn wire is the Internal Tin process as the volume fractions of Sn and Nb can be much higher than in the bronze process. Bronze with Sn contents above 15 wt % is too brittle to draw, while in the Internal Tin process, the Nb3Sn volume fraction can be as great as 60% or so, excluding any copper stabilizer that may be added.
In superconducting wire manufactured by the internal diffusion method, the Sn base metal material is disposed at the center of the module and, hence, the space between adjacent Nb3Sn filaments is as narrow as about a half of the spacing between such filaments arranged in accordance with the bronze method. For this reason the Nb base metal filaments tend to come into contact with each other to combine with each other when the superconducting wire precursor is heat-treated, thus resulting in an increase in the effective filament diameter (deff), which greatly influences the electrical characteristics of the superconducting wire.
The effective filament diameter is a measure of the functionality of the conductor. It represents the width of magnetization of the superconducting wire, and Jc represents the critical current density in these conditions. As a result, a problem arises that although the resulting superconducting wire suffers no problem with respect to DC current, it suffers a large hysteresis loss when a pulse current flows therein. A large effective filament diameter, deff, caused by actual large filament diameters or by filament bridging, produces large AC losses and poor field quality. The effective filament diameter is preferably maintained in the range of from about 5 xcexcmicrons to about 100 xcexcmicrons; preferably in the range of from about 5 xcexcmicrons to about 40 xcexcmicrons.
It is desirable to maximize the current density in the multifilament conductor. One method being used to significantly increase the current density in multifilament Nb3Sn is to increase the volume fraction of both Nb and Sn in various internal tin process composites while minimizing factors that cause a high deff.
Upon reaction with Sn to form Nb3Sn, the filaments in the Nb3Sn composites expand by about 39% in area. This expansion results in filament bridging such that the filaments act, when placed in swept magnet fields, as if they were essentially one. This can lead to instability of the conductor as a result of flux jumps and distorts the low field magnet field uniformity.
Target specifications set by the High Energy Physics community require the effective filament diameter in the Nb3Sn composites to be less than 40 microns, with current densities above 3000A/mm2 at 12 Tesla (T). This combination is presently unachievable without a method to minimize the filament bridging.
Thus, it is an object to provide a process for fabricating Nb3Sn wire that controls the amount of filament bridging.
It is a further object of this invention to increase the ratio of Nb3Sn to matrix in the conductor.
The invention provides a method of producing multifilament superconducting Nb3Sn wire with low bridging potential. To control the bridging of the filaments during reaction a ductile diffusion/reaction barrier is introduced between the filaments as a radial sheet. The barrier may be constructed of Ta, Va, a NbTa alloy or combinations of such. Other ductile materials such as a sandwich of Nb about the aforementioned elements are also useful.
These radial barriers may be allowed to react as the superconductor is formed if they possess low critical temperature and field characteristics. In one embodiment of the disclosure a circumferential barrier is also present. The preferred outer or circumferential barrier is Nb, which partially reacts and adds to the current density. Other suitable outer barriers are Ta, and Va. A pure Ta barrier will not react to form a superconductor while the others will form poor superconductors with low critical field characteristics.
The overlap of the barrier prevents the full circumference from reacting, which would defeat the radial barrier""s purpose. Additional radial barriers, as many as twelve [12] but preferably no more than six [6] and most preferably no more than three [3] may be utilized to further segment the filaments depending on the deff specification.
After reaction the filaments act with an effective diameter of not greater than the area represented by each pie segment. Hence the magnetization of the conductor can be controlled and designed to meet various specifications, such as that required for High Energy Physics accelerator magnets (40-60 microns).
As additional radial barriers are introduced the volume fractions of Nb and hence Nb3Sn are likely to be reduced such that a trade off in current density vs. magnetization has to be made. Optimally the volume fraction of Nb3Sn should be at least 50%; preferably at least 60%. The approximate loss that occurs per additional barrier is about 1.2% per barrier.